Carbon nanoarchitectures with ultrathin, conformal polymer coatings for electrochemical capacitors

ABSTRACT

A composite having an electroactive polymer coating on a porous carbon structure is disclosed. The composite may be used in capacitor electrodes. The composite may be made by self-limiting electropolymerization of a monomer on the carbon structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to composites that may be usedin energy storage devices.

2. Description of the Prior Art

Electrochemical capacitors (also denoted as supercapacitors orultracapacitors) are a class of energy-storage materials that offersignificant promise in bridging the performance gap between the highenergy density of batteries and the high power density derived-fromdielectric capacitors. Energy storage in an electrochemical capacitor isaccomplished by two principal mechanisms: double-layer capacitance andpseudocapacitance. Double-layer capacitance arises from the separationof charge that occurs at an electrified interface. With this mechanismthe capacitance is related to the active electrode surface area, withpractical capacitances in liquid electrolytes of 10-40 μF/cm².Electrochemical capacitors based on double-layer capacitance aretypically designed with high-surface-area carbon electrodes, includingcarbon aerogels, foams, and papers. (Frackowiak et al., Carbon, 39, 937(2001). All referenced patents and publications are incorporated byreference.) Carbon aerogels are particularly attractive due to theirhigh surface areas, high porosities, and excellent conductivities (>40S/cm) (Pekala et al., J. Non-Cryst. Solids, 225, 74 (1998)).

Pseudocapacitance broadly describes Faradaic reactions whose dischargeprofiles mimic those of double-layer capacitors. Because this mechanisminvolves true electron-transfer reactions and is not strictly limited tothe electrode surface, materials exhibiting pseudocapacitance often havehigher energy densities relative to double-layer capacitors. The mainclasses of materials being researched for their pseudocapacitance aretransition metal oxides, conductive nitrides, and conducting polymers.At present, some of the best candidates for electrochemical capacitorsare based on nanoscale forms of mixed ion-electron conducting metaloxides and hydrous metal oxides, such as RuO₂, which store charge via acation-electron insertion mechanism, as shown in equation (1).

Electrodes based on disordered, hydrous RuO₂ yield specific capacitancesas high as 720 F/g (Zheng et al., J. Electrochem. Soc., 142, 2699(1995)). The application of RuO₂ is limited however by the high costs ofthe ruthenium precursors.

Electronically conducting polymers store charge by a doping/de-dopingprocess where electronic state changes in the polymer are compensated bycation or anion incorporation from the supporting electrolyte. Examplesof relevant conducting polymers include polyaniline (equation (2)),polypyrrole, polythiophene, polyacetylene, and their derivatives.Because this Faradaic doping process occurs through the bulk volume ofthe polymer, high energy densities are accessible, as observed with themetal oxides. However, conducting polymers offer the advantage of lowercosts relative to those for noble metal oxides. A potential disadvantagefor conducting polymers is their somewhat lower conductivities (1-100S/cm) compared to carbon-based capacitors. The conductivity of suchpolymers also undergoes modulations as they are electrochemically cycledbetween the doped (conducting) and de-doped (insulating) state.Conducting polymer electrodes are typically fabricated byelectrodepositing thick (up to 10 μm) coatings onto carbon paper, thustheir electrical properties may restrict their overall rates of chargeand discharge for deep levels of de-doping (A. Rudge et al., J. PowerSources, 47, 89 (1994)).

SUMMARY OF THE INVENTION

The invention comprises a composite of a porous carbon structurecomprising a surface and pores, and an electroactive polymer coating onthe surface. The coating does not completely fill or obstruct a majorityof the pores.

The invention further comprises a capacitor comprising an anode, acathode, an electrolyte, and a current collector. The anode, thecathode, or both comprise the above composite. The current collector isin electrical contact with the composite.

The invention further comprises a method of forming a compositecomprising the steps of: providing porous carbon structure comprising asurface and pores; infiltrating the structure with a monomer which canform an electroactive polymer; and electropolymerizing the monomerforming a coating on the surface comprising the electroactive polymerwithout completely filling or obstructing a majority of the pores.

The invention further comprises a method of storing charge comprisingthe steps of: providing the above capacitor and charging the capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be readily obtainedby reference to the following Description of the Example Embodiments andthe accompanying drawings.

FIG. 1 schematically illustrates part of one electrode of a capacitorusing the composite of the invention.

FIG. 2 shows a comparison of cyclic voltammograms of a carbon aerogeland a poly(o-methoxyaniline)-coated carbon aerogel.

FIG. 3 shows a comparison of gravimetric and volumetric capacitances ofa carbon aerogel and a poly(o-methoxyaniline)-coated carbon aerogel.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

Conducting polymer/carbon aerogel hybrid architectures may be made byusing electropolymerization to apply ultrathin, conformal polymercoatings to pre-formed, highly conductive carbon aerogel electrodes.These nanostructured hybrids may be designed as electrode materials forhigh-energy-density electrochemical capacitors. Carbon aerogels (alsodenoted as carbon nanofoams) may serve as high surface area, highlyporous, conductive electrode structures. Ultrathin polymer coatings canbe applied to these structures by electropolymerization usingexperimental conditions whereby polymer growth is self-limiting. Usingthis approach, polymers can be grown on the three-dimensional carbonelectrode surface without occluding or filling the porosity of thecarbon aerogel. The resulting hybrid structures may exhibit enhancedgravimetric and volumetric capacitance when electrochemically cycled inaqueous acid electrolytes.

FIG. 1 schematically illustrates one electrode of a capacitor using thecomposite of the invention. The carbon aerogel 10 has a coating ofconducting polymer 20, and is electrically connected to a currentcollector 30. An electrolyte 40 permeates the composite and is incontact with the entire coating.

Any porous carbon structure, or combinations thereof may be used to makethe composite including microporous, mesoporous, and macroporous formspossessing a continuous pore network. Suitable forms of porous carboninclude, but are not limited to carbon aerogel, carbon nanofoarn, andtemplated mesoporous carbon. Carbon aerogels may be prepared accordingto methods disclosed in Bock et al., J. Non-Cryst. Solids, 225, 69(1998), Pekala et al., U.S. Pat. No. 4,873,218, and Pekala et al., U.S.Pat. No. 4,997,804, or by other suitable methods. A suitable averagepore size in the porous carbon structure is about 2 nm to about 1 μm indiameter.

The polymer may be any electroactive polymer, or combinations thereof.Suitable electroactive polymers include, but are not limited to, aconductive polymer, a redox polymer, a polyaniline, a polyarylamine, apolypyrrole, a polythiophene, a polyacetylene, and their respectivederivatives.

A monomer that polymerizes to an electroactive polymer may be used toform the polymer coating on the surface of the carbon. A suitable methodof infiltrating the carbon with the monomer is to immerse the carbon ina solution of the monomer. A typical monomer solution may contain 10 mMmonomer, 0.2 M Na₂SO₄ as an electrolyte, and 50 mM of a citrate bufferto maintain the solution pH at a value of 4.7.

Electropolymerization may be performed by flowing a current through thecarbon and monomer solution. If the electropolymerization is done at ahigh enough pH, then the resulting polymer coating may be nonconducting.This property will limit the thickness of the coating such that amajority of the pores are not completely filled or obstructed. Amajority means at least 50%, but may be as much as 70%, 90%, 99%, ormore. An obstructed pore is one that is not completely filled withpolymer, but is one that has blocked openings such that an electrolytecould not enter the pore. It is to be understood that some pores couldend up being filled or obstructed due to variations in coating thicknessand pore size, and that such would be within the scope of the invention.

Electropolymerization schemes involving such monomers aso-phenylenediamine, aniline, o-aminophenol, and o-methoxyaniline may beused. When performed in aqueous electrolytes with pH>1, theelectro-oxidization of these monomers can result in ultrathin (<10 nm)coatings of polymer at the electrode surface. Film formation can beself-limiting due to the poor electronic conductivity and minimalswelling of the growing polymer. By choosing self-limiting growthconditions, conformal polymer coatings can be applied over the entireelectroactive area of the high-surface-area electrode, while notoccluding or filling the through-porous structure of the carbon aerogel.Although these polymer films may be grown under solution conditionswhere they are insulating or poorly conductive, when transferred to anacidic aqueous electrolyte the polymers can be converted to theirelectroactive state and contribute a significant pseudocapacitance tothe hybrid structure. Some examples of monomers that lead to suchself-limited polymer coatings are shown in equation (3).

A capacitor electrode may be made using the composite. The composite isplaced in electrical contact with a current collector such as a lead.The composite is at least partly immersed in an electrolyte, such assulfuric acid, aqueous acid, or a protonic ionic liquid. A secondelectrode, which may also comprise the composite, is also placed incontact with the electrolyte. The capacitor may then be charged.

The composite may offer several performance enhancements when used as anelectrochemical capacitor. Adding the polymer component to the carbonaerogel can increase the specific capacitance of the hybrid through theFaradaic pseudocapacitance of the polymer, even after accounting for theadditional mass of the polymer. These hybrid structures offer evengreater enhancement when the volumetric capacitance is considered. Dueto the porous structure of the carbon aerogel and the limited growth ofthe polymer, the bulk volume of carbon aerogel structure is virtuallyunaltered by the addition of the polymer component. Thus, all additionalcapacitance from the conducting polymer can contribute directly toincreasing the volumetric capacitance of the hybrid.

Because the composites are designed to retain the high porosity of theinitial carbon aerogel, substantial gains in total capacitance areachieved without significantly sacrificing charge-discharge ratecapability. In this configuration the carbon∥polymer∥electrolyteinterface may not exceed ˜10 nm across, and performance limitations dueto the conductor-to-insulator transition of the polymer can beminimized. This configuration can also exploit the short transportlengths of dopant ions from the infiltrated electrolyte into theultrathin polymer coating.

The use of self-limiting electropolymerization conditions describedherein may not require the careful electrochemical control that wouldotherwise be necessary to coat a highly porous electrode while stillretaining the porous architecture. For example, the use of a shortpotential pulse under non-limiting conditions may be used, but mayresult in poor control of film thickness. This protocol may result,though not necessarily, in filling or obstructing an unacceptable numberof pores.

Having described the invention, the following examples are given toillustrate specific applications of the invention. These specificexamples are not intended to limit the scope of the invention describedin this application.

EXAMPLE 1

Electropolymerization—Carbon aerogels were prepared according toestablished procedures (V. Bock et al., J. Non-Cryst. Solids, 225, 69(1998)). Monolithic pieces of carbon aerogel (2-4 mg) were incorporatedinto a stainless-steel basket electrode, which provided for electricalcontact between the aerogel and a potentiostat/galvanostat. Thisassembly was equilibrated in a hydration chamber for 24 hours to wet thecarbon surfaces, then immersed in an initial electrolyte and evacuatedfor 2 hr. This procedure was used to efficiently infiltrate the porouscarbon structure with electrolyte. The equilibrated electrode assemblywas immersed in an electrolyte containing 20 mM oxidizable monomer, suchas o-methoxyaniline, 0.2 M Na₂SO₄, and 50 mM citrate buffer.Polymerization of the monomer was initiated using a number ofelectrochemical techniques including voltammetric, potentiostatic,galvanostatic, potential-pulse, and current-pulse methods. For the classof aniline-based monomers studied here, the oxidation of the monomersoccurs at potentials between 0.2 and 0.6 V. The electropolymerizationstep was always performed under conditions where the polymer growth wasself-limiting and resulted in polymer coatings less than 10-nm thick.For electropolymerization from arylamine monomers this typicallyrequires an aqueous monomer solution with pH>1. When these self-limitingconditions are employed, these polymer coatings did not exceed ˜15 nm inthickness, no matter what electrochemical method was applied. Theresults shown in FIGS. 2 and 3 were obtained from a composite made byapplying a series of potential pulses: pulsing at 1.0 V for 15 secondsfollowed by a pulse at 0 V for 60 seconds, with a total of 200 pulsecycles. The solution for the polymer deposition was 20 mMo-methoxyaniline in 0.2 M Na₂SO₄, buffered at pH 4.7.

EXAMPLE 2

Electrochemical characterization—Following the electropolymerizationstep, the electrode assembly was transferred to an aqueous acidelectrolyte, typically 0.1 M H₂SO₄, for electrochemicalcharacterization. Under these acidic conditions the ultrathin polymercoating became electroactive. The overall electrochemical capacitance ofthe hybrid structure was increased by the pseudocapacitance of theactivated polymer component. Galvanostatic (constant-current)charge-discharge cycling was used to assess the capacitance of theelectrode as a function of the current load and the number of cycles.

FIG. 2 shows a comparison of cyclic voltammograms of a carbon aerogeland a poly(o-methoxyaniline)-coated carbon aerogel. This comparisonshows the presence of the peaks for the oxidation and reduction of thePOMA coating at the carbon aerogel electrode.

With a carbon aerogel coated with poly(o-methoxyaniline), POMA, thegravimetric capacitance was increased ˜35% for a current load of 0.5A·g⁻¹ and ˜100% for a higher current load of 4 A·g⁻¹ (FIG. 3 a). Forthese same current loads the volumetric capacitance was enhanced evengreater, with increases of ˜90% and 270%, respectively (see FIG. 3 b).The gravimetric capacitance values for the POMA-carbon hybrid took intoaccount the additional mass of the polymer component, ˜26 wt % of thecomposite. The volumetric capacitance values were derived based on theenvelope volume of the initial carbon aerogel, which was unaltered bythe addition of the polymer coating.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that the claimed invention may be practiced otherwise than asspecifically described.

1. A composite comprising a porous carbon structure comprising a surfaceand pores; and a coating on the surface comprising an electroactivepolymer; wherein the coating does not completely fill or obstruct amajority of the pores; wherein the coating is formed by self-limitingelectropolymerization and does not exceed about 15 nanometers inthickness.
 2. The composite of claim 1, wherein the structure is acarbon aerogel.
 3. The composite of claim 1, wherein the structure isselected from the group consisting of carbon nanofoam and templatedmesoporous carbon.
 4. The composite of claim 1, wherein the pores havean average diameter of from about 2 nm to about 1 μm.
 5. The compositeof claim 1, wherein the polymer is a conductive polymer.
 6. Thecomposite of claim 1, wherein the polymer is a polyaniline or derivativethereof.
 7. The composite of claim 1, wherein the polymer is selectedfrom group consisting of a redox polymer, a polyarylamine, apolypyrrole, polyacetylene, a polythiophene, and derivatives thereof. 8.The composite of claim 1, wherein the coating has a thickness of no morethan about 10 nm.
 9. A capacitor comprising an anode, a cathode, and anelectrolyte, wherein the anode, the cathode, or both comprise: acomposite comprising a porous carbon structure comprising a surface andpores; and a coating on the surface comprising an electroactive polymer;wherein the coating does not completely fill or obstruct a majority ofthe pores; and wherein the coating is formed by self-limitingelectropolymerization and does not exceed about 15 nanometers inthickness; and a current collector in electrical contact with thecomposite.
 10. The capacitor of claim 9, wherein the structure is acarbon aerogel.
 11. The capacitor of claim 9, wherein the structure isselected from the group consisting of carbon nanofoam and templatedmesoporous carbon.
 12. The capacitor of claim 9, wherein the pores havean average diameter of from about 2 nm to about 1 μm.
 13. The capacitorof claim 9, wherein the polymer is a conductive polymer.
 14. Thecapacitor of claim 9, wherein the polymer is a polyaniline or derivativethereof.
 15. The capacitor of claim 9, wherein the polymer is selectedfrom group consisting of a redox polymer, a polyarylamine, apolypyrrole, polyacetylene, a polythiophene, and derivatives thereof.16. The capacitor of claim 9, wherein the coating has a thickness of nomore than about 10 nm.
 17. The capacitor of claim 9, wherein theelectrolyte comprises sulfuric acid.
 18. The capacitor of claim 9,wherein the electrolyte comprises a liquid selected from the groupconsisting of an aqueous acid and a protonic ionic liquid.